1. Field of the Invention
The present invention is directed to devices for receiving light signals, in particular (but not exclusively) signals transmitted by optical fiber and which are converted into electric signals by a photodiode of the PIN type. More particularly, the present invention is directed to preamplifiers of the transimpedance type intended to amplify such electric signals.
2. Discussion of the Background
By taking as an example a case of the transmission of data by optical fiber, FIG. 1 diagrammatically illustrates the use of this technique.
As shown in FIG. 1, data is transmitted in the form of light signals by an optical fiber 1 to a receiver 2. Receiver 2 comprises a light-sensitive device 3 and an amplifier, or more precisely a preamplifier 4. Light-sensitive device 3 converts the light signals into electric signals which are applied to the input of preamplifier 4. The corresponding output electric signals, delivered by preamplifier 4, are then transmitted to an acquisition device 5 to be processed.
Light-sensitive device 3 can be formed of, for example, a phototransistor or a photodiode, and very often it is formed of a photodiode of the PIN type. When photodiode 3 is formed of a PIN type photodiode, photodiode 3 is back-biased, and it can be considered as low level, like a current generator shunted by a stray capacitance CP, as shown by the dotted lines in FIG. 1. It should be noted that the light-sensitive element 3 can be of any type since it is inserted in a circuit making it possible to produce a current generator producing a current i reflecting the light signals received.
Since preamplifier 4 is at the head of the receiving chain, its characteristics largely determine the overall performances of receiver 2. It is generally desired for preamplifier 4 to have sufficient (often considerable) gain with a wide passband and an equivalent low noise brought as an input to cause as little deterioration as possible to the intrinsic sensitivity of the sensor, i.e., the light-sensitive element 3. Preamplifier 4 is of the transimpedance type, in which gain Z is provided by the ratio of output voltage VS to input current i delivered by light-sensitive element 3 (Z=VS/i).
Transimpedance preamplifiers are currently used to amplify signals delivered by photodiodes of the PIN type (P conductivity, intrinsic, N conductivity); the description of such a preamplifier is found in particular in an article by Robert G. Meyer and Robert Alain Blauschild "A Wide-Band Low-Noise Monolithic Transimpedance Amplifier" published in IEEE Journal of Solid-State Circuits, Vol. SC-21, No. 4, Aug. 1986, pages 530 to 533.
FIG. 2 diagrammatically shows the basic circuit according to which, in the prior art, transimpedance preamplifiers are joined to a PIN photodiode, and in the case in particular where the PIN photodiode receives signals transmitted by an optical fiber.
Photodiode 3 is represented in the form of a current generator (shunted by stray capacitance CP). Amplifier 4 is a large-gain wide-band voltage amplifier, with negative feedback by a resistance R. Light-sensitive element or current generator 3 is connected to the negative input "-" of the amplifier 4.
The circuit shown in FIG. 2 constitutes a basic circuit broadly described with different optimizations. But in all cases, one of the significant problems in such a circuit resides in the difficulty that there is in obtaining a fairly wide passband BW (required by a high data flow) while maintaining a sufficient phase margin to assure a good stability of the amplifier 4.
It should be noted that the frequency response of the circuit illustrated in FIG. 2 is largely governed, on the one hand, by the stray capacitane CP of the PIN diode, and on the other hand, by an input capacitance CR of the amplifier (this capacitance CR is represented by the dotted lines as being a stray capacitance associated with negative feedback resistance R). A very great sensitivity of the passband, and therefore of the stability, to the value of 5 stray capacitance CP of the PIN diode results from this. This detail is particularly troublesome when it is desired to construct as universal a preamplifier as possible available, i.e., a preamplifier which is able to operate with various types of light-sensitive elements and therefore with various values of stray capacitance CP.
More precisely, passband BW of the amplifier also depends on stray capacitance CR of resistance R, as indicated in the following equation: EQU BW=1/2 (CR.multidot.R+CP.multidot.R/G) (1)
where G is the gain of the amplifier.
Now, in practice, stray capacitance CR reduces the potential passband, and this all the more so since, according to a general step, it is desired to increase R to minimize the noise brought back as an input.
It should be noted that another drawback of a preamplifier stage produced using an amplifier as illustrated in FIG. 2 is that to keep the effectiveness of the negative feedback as far away as possible in frequency, it is desired to make the output impedance of this amplifier very low, which is achieved at the cost of a large consumption of current on the output followers (not shown) that such an amplifier comprises in a standard way.